Many methods exist for objectively measuring the perceived loudness of audio signals. Examples of methods include A, B and C weighted power measures as well as psychoacoustic models of loudness such as “Acoustics—Method for calculating loudness level,” ISO 532 (1975) and said PCT/US2005/038579 application. Weighted power measures operate by taking the input audio signal, applying a known filter that emphasizes more perceptibly sensitive frequencies while deemphasizing less perceptibly sensitive frequencies, and then averaging the power of the filtered signal over a predetermined length of time. Psychoacoustic methods are typically more complex and aim to better model the workings of the human ear. They divide the signal into frequency bands that mimic the frequency response and sensitivity of the ear, and then manipulate and integrate these bands while taking into account psychoacoustic phenomenon such as frequency and temporal masking, as well as the non-linear perception of loudness with varying signal intensity. The aim of all methods is to derive a numerical measurement that closely matches the subjective impression of the audio signal.
Accurate modeling of the non-linearity of the human auditory system forms the basis of perceptual models of loudness. In the 1930's, Fletcher and Munson found that the relative change in sensitivity decreased as the level of sound increased. In the 1950's, Zwicker and Stevens built on the work of Fletcher and Munson and developed more accurate and realistic models. FIG. 1, published by Zwicker, shows the growth of loudness of both a 1 kHz tone and uniform exciting noise (UEN, noise with equal power in all critical bands). For a signal level below what is often termed the “hearing threshold,” no loudness is perceived. Above this threshold, there is a quick rise in perceived loudness up to an asymptote where loudness grows linearly with signal level. Where FIG. 1 shows the non-linear behavior for a 1 kHz tone, the equal loudness contours of ISO 226 in FIG. 2 show the same behavior but as a function of frequency for sinusoidal tones. The contour lines, at increments of 10 phon, show the sound pressure levels across frequency that the human ear perceives as equally loud. The lowest line represents the “hearing threshold” as a function of frequency. At lower levels the lines of equal loudness compress closer together such that relatively smaller changes in sound pressure level cause more significant changes in perceived loudness than at higher levels.
The non-linear and frequency varying behavior of the human auditory system has a direct impact on the perceived timbre and imaging of audio signals. A complex, wideband audio signal, for example music, presented at a particular sound pressure level is perceived as having a particular spectral balance or timbre. If the same audio signal is presented at a different sound pressure level and, as shown in FIG. 2, the growth of perceived loudness is different for different frequencies, the perceived spectral balance or timbre of the audio signal will be different. A complex, wideband multichannel audio signal, presented over multiple loudspeakers, is also perceived as having a particular spatial balance. Spatial balance refers to the impression of the location of sound elements in the mix as well as the overall diffuseness of the mix due to the relative level of audio signals between two or more loudspeakers. If the same multichannel audio signal is presented at a different overall sound pressure level, the non-linear growth in perceived loudness and differing growth of loudness across frequency leads to a change in the perceived spatial balance of the multichannel audio signal. This is especially apparent when there is a significant difference in level between channels. Quieter channels will be affected differently to louder channels which, for example, can lead to quiet channels dropping below the hearing threshold and audibly disappearing when the overall level is reduced.
In many situations there is a desire to adjust or scale the perceived loudness of an audio signal. The most obvious examples are the traditional volume or level controls that appear on many devices including consumer music players, home theater receiver/amplifiers and professional mixing consoles. This simple volume or level control gain adjusts the audio signal without any consideration of the human auditory system and resulting change in perceived timbre and spatial balance.
More recently Seefeldt et. al (said WO 2004/111994 A2 application) and Seefeldt (said PCT/US2005/038579 application) have disclosed inventions, aspects of which enable accurate scaling of the perceived loudness of a monophonic audio signal and, depending on whether implementations thereof are wideband or multiband, maintain the perceived timbre. According to aspects of such inventions, a desired loudness scaling or target loudness may be achieved by, in essence, inverting the loudness measurement model and calculating either a wideband gain or multiband gains that can be applied to the audio signal.
While such approaches solve the problem of adjusting the loudness of a monophonic audio signal, the question still remains of how to adjust the loudness of a multichannel audio signal.
Multichannel loudness is typically calculated as a function of the sum of the power in each channel. For weighted power methods such as the A, B and C weighted measures mentioned above, the multichannel loudness is a simple sum of the weighted power in each channel. Commonly for psychoacoustic models of loudness, a critical band power spectrum or excitation spectrum is first calculated for each channel and the excitation spectrums are then summed across all the channels to create a single excitation spectrum. Each excitation band is passed through a non-linearity, such as FIG. 1, to create a measure of loudness per band, known as specific loudness, and the specific loudness is summed across frequency to calculate a single, wideband loudness value. For both weighted power and psychoacoustics methods, the function of the sum of the power in each channel may include additional per channel weightings to take into account head related transfer function (HRTF) effects.
Because the loudness of a multichannel signal can be calculated relatively simply, it is possible to calculate a single gain that, when applied to all channels, causes an overall desired change in loudness. However, this single gain may have undesirable effects on other attributes of the multichannel presentation. If differences exist in the relative signal levels between channels in the multichannel presentation and if all channels are scaled by the same gain, quieter channels will have a larger perceived change in their loudness than louder channels. This may cause a change in the perceived spatial balance that is worst when some channels fall below the threshold of hearing. For example, in many 5.1 audio mixes for film, the front channels contain signals of a significantly higher level than the surround channels. The center channel in particular is generally used to reproduce dialogue. The lower level surround channels, however, may contain signals that create a sense of diffuseness in the mix. For example, they may contain the reverberant portion of the dialogue in order to simulate the effect of someone speaking in a large room. As the loudness of such a signal is decreased by applying the same gain to all channels, the surround channels decrease in loudness more rapidly than the front channels, eventually falling below the threshold of hearing. The result is a significant collapse in the intended diffuse spatial balance.
According to aspects of the present invention, a desired scaling in the overall perceived loudness of a multichannel presentation may be achieved to a desired accuracy, while retaining, to a desired accuracy, the relative perceived loudness among channels in order to preserve a perceived spatial balance or timbre.